The invention relates to the connection of a marine riser between a wellhead on the seafloor and a pressure-controlling valve assembly (tree) upon a floating platform at the sea's surface. The platform may be used for the production of hydrocarbons (such as a SPAR, Deep Draft Caisson Vessel, or Tension Leg Platform), or for drilling into hydrocarbon reservoirs. The ends of the marine riser typically possess some physical features for connection and reaction of the loads between these widely separated parts. One such feature is termed a “stress joint”, a segment of the riser with a varying, specially shaped cross-section for a smooth transfer of load and deflection to the terminus of the riser with minimum stresses. Another such feature is one or more parts or specially shaped surfaces that are attached, or can be attached, to the terminus of the riser that allow for a remotely operated connection to be made. The requirements of any connection features are demanding. Though the stress joint and riser flex significantly, there are still residual bending moments and tensions that must be transmitted through the connection in order to keep it securely water- or gas-tight. In addition, the connecting features must enable mate-up and demating between the riser's lower terminus and the wellhead. Such mating must occur remotely, underwater, and sometimes in poor conditions. Back-up and fail-safe functions may be necessary.
As a result, the various connection features are typically embodied in an equipment assembly attached to the riser's lower terminus and called a “subsea tieback connector”. The assembly is composed of a number of robust, highly engineered components. Historically, many such connector assemblies were “female”, swallowing a specially contoured surface on the exterior of the wellhead (making it the “male”), such as a mandrel or hub. The connector parts could then be made as large as needed in order to carry the load and execute their numerous functions.
On any floating hydrocarbon production platform, space and buoyancy are limited. One method for supporting the weight and tension of a marine riser is with individual flotation vessels, termed “air cans”. The air-cans may be permanently attached to the riser along a significant part of its length (termed “integral”), or only at a single point (termed “non-integral”). In the latter case, all but the uppermost part of the riser string must drift through a passage formed in the center of the air-cans. The drifting parts include the lower terminus and any features for the lower connection.
To this end, it is desirable for the lower terminus and any connection features to be as small a diameter as possible, so that the opening in the air can is likewise as small as possible, in order to maximize the amount of flotation afforded by said air-can.
The design challenge is to enable the necessarily robust connection features, while keeping the overall diameter small. This has resulted in prior art with complex designs, costly high-performance materials, costly specially shaped parts, and/or overly sensitive operation. And typically the connection strength is still limited relative to a connector not so constrained.
An alternative to squeezing all the connection features into the restricted air can diameter, is to have only the bare minimum of said features attached to the lower terminus. The remaining features must then be provided in a separate assembly. The features on the lower terminus may be limited to a special profile formed on the exterior, similar to that on the wellhead.
The separate assembly must be independently placed subsea in the vicinity of the wellhead. The placement may be executed at any time by a small boat and submersible ROV (Remotely Operated Vehicle) independent of the operations on the platform. Said assembly must enable a connection between essentially three separate members: the riser's lower terminus with minimized connection features, the connector assembly itself, and the wellhead.
An ideal connector for this application has only one sealing joint, one leak path, one set of functions, can be independently pre-placed and operated by an ROV, withstands very high loads, and needs a passage through the air cans no larger than the minimum required stress joint. To this end, the following invention—a tieback connector for subsea tieback—is applied.